Connect with us

Hi, what are you looking for?

SecurityWeekSecurityWeek

Application Security

Firefox Users Fingerprinted via Cached Intermediate CA Certificates

An attacker can discover various details about Firefox users due to the manner in which the browser caches intermediate CA certificates, a researcher has discovered.

An attacker can discover various details about Firefox users due to the manner in which the browser caches intermediate CA certificates, a researcher has discovered.

When the server doesn’t deliver the complete certificate chain, Firefox loads the website if the intermediate CA certificate is cached, security researcher Alexander Klink discovered. By determining which websites use the same intermediate, an attacker could figure out some details about the user, the researcher says.

Normally, root Certificate Authorities (CAs) don’t use the main root certificate to secure connections, but generate intermediate certificates instead. Webservers use these intermediates to generate certificates for each user, and deliver these (server certificates) to the browser along with the intermediate CA certificate when establishing a connection.

When a server is misconfigured, only the server certificate is sent, which should result in the user getting an error instead of the website. However, if the intermediate CA certificate has been already cached, the user will be able to connect. While Chrome and Internet Explorer don’t rely on the entire chain to deliver a website, Firefox does, but uses cached CAs even when in Private Mode, the researcher has discovered.

According to Klink, an attacker could use this knowledge to determine specific details about targeted users, based on the intermediate CA certificates cached by their browsers. However, these details would be limited to geolocation, maybe browsing habits, and whether the victim’s browser runs in a sandbox (which would lack cached certificates). The attacker could sell this information to advertising companies or could leverage it to deliver specific content to the targeted users.

“In addition to the purely »statistical« view of having a fingerprint with a sequence of n bits representing the cache status for each tested CA, the fingerprint also contains additional semantic information. Certain CAs have customers mostly in one country or region, or might have even more specific use-cases which lets you infer even more information − i.e. a user who has the »Deutsche Bundestag CA« cached is most probably located in Germany and probably at least somewhat interested in politics,” the researcher explains.

Klink also notes that he contacted Mozilla on the matter in January, but that there are no details on what course of action the organization will take. The “cleanest solution” would be to avoid connecting to incorrectly configured servers, even if the intermediate CA is cached, but “Mozilla is reluctant to implement that without knowing the impact,” the researcher says.

Users can stay protected by regularly cleaning up their profiles, by creating new ones, by cleaning up existing ones from the Firefox UI, or by using the certutil command line tool. They can also block third-party requests with an addon, mainly because “the attack obviously needs to make (a lot of) third-party requests,” Klink concludes.

Advertisement. Scroll to continue reading.

Related: GoDaddy Revokes Nearly 9,000 SSL Certificates

Related: CASC Releases Minimum Requirements for Code Signing Certificates

Related: Chrome’s Certificate Transparency to Become Mandatory

Written By

Ionut Arghire is an international correspondent for SecurityWeek.

Click to comment

Trending

Daily Briefing Newsletter

Subscribe to the SecurityWeek Email Briefing to stay informed on the latest threats, trends, and technology, along with insightful columns from industry experts.

Join the session as we discuss the challenges and best practices for cybersecurity leaders managing cloud identities.

Register

SecurityWeek’s Ransomware Resilience and Recovery Summit helps businesses to plan, prepare, and recover from a ransomware incident.

Register

Expert Insights

Related Content

Application Security

Cycode, a startup that provides solutions for protecting software source code, emerged from stealth mode on Tuesday with $4.6 million in seed funding.

Application Security

Virtualization technology giant VMware on Tuesday shipped urgent updates to fix a trio of security problems in multiple software products, including a virtual machine...

Application Security

Fortinet on Monday issued an emergency patch to cover a severe vulnerability in its FortiOS SSL-VPN product, warning that hackers have already exploited the...

Application Security

PayPal is alerting roughly 35,000 individuals that their accounts have been targeted in a credential stuffing campaign.

Application Security

GitHub this week announced the revocation of three certificates used for the GitHub Desktop and Atom applications.

Application Security

A CSRF vulnerability in the source control management (SCM) service Kudu could be exploited to achieve remote code execution in multiple Azure services.

Application Security

Drupal released updates that resolve four vulnerabilities in Drupal core and three plugins.